1. Field of the Invention
This invention generally relates to an aluminum alloy sheet for a magnetic disk substrate, an aluminum alloy clad sheet for a magnetic disk substrate and their manufacturing method, and more particularly, to an aluminum alloy sheet for a magnetic disk substrate, an aluminum alloy clad sheet for a magnetic disk substrate and their manufacturing method, in which there is less formation of micropits or nodules (i.e., semi-spherical protrusions) in case of plating a substrate to enable a satisfactory coat with a magnetic substance, and also an increase in magnetic disk storage memory capacity.
2. Description of the Prior Art
Recently introduced magnetic disks have attempted to meet demands for increase in storage memory capacity and density in order to answer the needs of multimedia or the like, and show a tendency to reduce the magnetic area per one bit of a magnetic disk of reduced size, and to narrow the distance between a magnetic head and a magnetic disk.
Therefore, the following characteristics have recently become required for a magnetic disk substrate.
(1) It should be made of light weight aluminum alloys of non heat-treatment type (i.e., non precipitation-hardening type), and show strength sufficient to bear various works and high-speed rotation in use. PA1 (2) It should be more lightweight, and have a satisfactory specular surface free from any micropits or the like formed by means of grinding. PA1 (3) The surface resulting from plating the substrate should be smooth and free from any surface defects such as micropits. PA1 (4) A plated substrate layer should show excellent adhesion to the surface material of the magnetic disk substrate. PA1 a composition consisting of, by percent by weight, 2.0 to 6.0% of Mg, 0.05 to 0.15% of Cu, 0.10 to 0.30% of Zn, and 0.05 to 0.12% of Zr, PA1 wherein the contents of Cu, Zn and Zr satisfy the expression: 0.15%.ltoreq.2Cu+6Zr-3Zn.ltoreq.0.32% (provided that Cu, Zr and Zn in the above expression respectively represent their contents in terms of percent by weight); and PA1 the above composition further consists of one or both of 0.01 or more to less than 0.05% of Cr and 0.01 or more to less than 0.05% of Mn, impurities consisting of Si, Fe and Ti which are respectively regulated to be 0.05% or less, 0.05% or less and 0.02% or less, and other inevitable impurity elements respectively regulated to be 0.02% or less, with the remainder being Al. PA1 a step of manufacturing a slab by melting and casting an aluminum alloy having the composition as described above; PA1 a step of hot-rolling the slab into an aluminum alloy sheet; PA1 a step of cold-rolling the hot-rolled aluminum alloy sheet; and PA1 a step of finally annealing the cold-rolled aluminum alloy sheet; PA1 wherein cooling after the hot rolling, cooling after intermediate annealing carried out in the course of the cold rolling and cooling after the final annealing are respectively carried out at a temperature in the range of 240 to 150.degree. and at a rate in the range of 30 to 600.degree. C./hr; and PA1 the cold rolling is carried out three or more times at a rolling reduction of 20% or more on condition that the total rolling reduction is 60% or more. PA1 hot-rolling, cold-rolling and finally annealing a composite material a or b as described below, PA1 wherein cooling after the hot rolling, cooling after intermediate annealing carried out in the course of the cold rolling and cooling after the final annealing are respectively carried out at a temperature in the range of 240 to 150.degree. C. and at a rate in the range of 30 to 600.degree. C./hr; and PA1 the cold rolling is carried out three or more times at a rolling reduction of 20% or more on condition that the total rolling reduction is 60% or more. PA1 a: a composite material, in which one or both surfaces of a core material consisting of less than 3.0% by weight of Zn with the remainder being Al and inevitable impurity elements are clad with a surface material consisting of the aluminum alloy for a magnetic disk substrate according to the first embodiment PA1 b: a composite material, in which one or both surfaces of a core material consisting of less than 3.0% by weight of Zn with the remainder being Al and inevitable impurity elements are clad with a surface material consisting of the aluminum alloy for a magnetic disk substrate according to the first embodiment, and the ratio (Fb/Fs) of F value (Fb) of the core material to F value (Fs) of the surface material is 0.6 or more when F is given by the following expression: F=Si+Fe+2Cu+2Mn+3Mg+0.5Zn (provided that Si, Fe, Cu, Mn, Mg and Zn in the above expression respectively represent their contents in terms of percent by weight).
For a magnetic disk substrate satisfying the above characteristics, use is made of JIS A5086 aluminum alloy (having a composition consisting of, by percent by weight, 3.5 to 4.5% of Mg, 0.5% or less of Fe, 0.4% or less of Si, 0.2 to 0.7% of Mn, 0.05 to 0.25% of Cr, 0.10% or less of Cu, 0.15% or less of Ti, and 0.25% or less of Zn, with the remainder being Al), in which the contents of impurity elements such as Fe and Si causing micropits or the like are regulated.
The magnetic disk substrate is manufactured by the steps of hot-rolling a slab of the above alloy, which is manufactured by a semi-continuous casting method, into a rolled material, subsequently cold-rolling this rolled material while carrying out annealing in the course of cold rolling, then manufacturing the rolled material into a disk by means of stamping, giving this disk a pretreatment including cutting, grinding, polishing, degreasing, etching and treatment with zincate, further giving this disk a substrate treatment including electroless plating with a hard non-magnetic metal such as Ni-P, then polishing the disk thus treated, and thereafter coating the polished disk with a magnetic film such as Co-Ni-P alloy by means of sputtering.
However, the problem with the magnetic disk substrate made of JIS-A-5086 alloy described above is that its electroless-plated surface does not show satisfactory smoothness.
Namely, in the case where the rolled material is manufactured into the disk by means of stamping, and the pretreatment including cutting, grinding, polishing and treatment with zincate or the like is given to the disk, intermetallic compound particles contained in the alloy are left as protrusions on the disk surface, or fall off the disk surface, and as a result, nodules (i.e., semi-spherical protrusions) or micropits are formed on the disk surface resulting from plating its substrate.
For instance, when the intermetallic compound particles protrude from the pretreated disk surface, the protrudent intermetallic compound portions are plated and left as nodules. When the intermetallic compound particles fall off the disk surface at the stage of the pretreatment described above, zincate preferentially grows up in a convex shape in the depressions left by the intermetallic compound particles, in the treatment with zincate. Thus, the convex zincate portions are plated and left as nodules, or the depressions left by the intermetallic compound particles remain as micropits after plating.
If an electroless-plated layer on the disk has a large thickness (i.e., 20 .mu.m as in the prior art), the nodules or micropits as described above frequently disappear during polishing subsequent to electroless plating, and offer no problem. However, since the thickness of a plated layer on the disk has recently been reduced (i.e., down to 14 .mu.m or less), nodules or micropits are left on the disk surface even after polishing, and the surface smoothness of the magnetic disk substrate is degraded.
The nodules left on the surface of the magnetic disk substrate collide against a magnetic head, and cause a head crush. The micropits left on the surface of the magnetic disk substrate cause an electric error.
It is conceivable that the nodules or micropits in the magnetic disk substrate can be reduced if a high-purity aluminum material (i.e., purity of 99.99% by weight) containing less impurity elements such as Si and Fe, for instance, is used as the material of the magnetic disk substrate. However, the problem with the use of an aluminum material having purity of 99.99% by weight, instead of an aluminum material having purity of approximately 99.9% by weight, for instance, is that the former is particularly more expensive than the latter, and is limited in its distribution.
Measures to reduce the number or size of intermetallic compound particles in an aluminum alloy sheet have been devised for the purpose of preventing the nodules or micropits as described above from being formed on the magnetic disk substrate. However, such measures have not always attained satisfactory results.
The present inventors have already proposed an aluminum alloy sheet for a magnetic disk substrate, in which a plated substrate layer (i.e., a layer electroless-plated with Ni--P alloy) having less surface defects is obtained by regulating the kinds and contents of alloy elements (See Japanese Patent Laid-open No. Hei 2-97639, U.S. Pat. No. 5,017,337). Although this aluminum alloy sheet for a magnetic disk substrate has been already put to practical use, and is rated highly, such an aluminum alloy sheet has some problems in that it is not possible to provide a large storage memory capacity magnetic disk, i.e. having as much capacity as the latest 2GB/pc. or more.
One of the problems is that recently magnetic heads have been set to a smaller glide height (reduction of glide height is made from 1.5 to 1.0 micro inches, for instance) in order to attain the increase in storage memory capacity of a magnetic disk, and as a result, even fine nodules easily collide against the magnetic head.
Another problem is that in polishing a disk, i.e. in the so-called light polishing, a reduced polishing load has been employed to prevent shear drop from occurring at the end of the disk and to carry out economical polishing, and as a result, more nodules or micropits have been easily left on the disk surface.
A further problem is that the magnetic area per one bit has been made smaller by the need for increasing the storage memory capacity of a magnetic disk, and as a result, even fine micropits on the outermost surface layer of the magnetic disk resulting from plating its substrate easily cause electric errors.
Further, for the purpose of enhancing the strength of a magnetic disk substrate, a magnetic disk substrate has been proposed in which a core material consisting of Al--Mg--Zn alloy (JIS 7000 alloy), one of age hardening aluminum alloys, for instance, is clad with Al--Mg alloy (JIS 5000 alloy) having sufficient plating properties, as a surface material (See Japanese Patent Laid-open Nos. Hei 5-9633 and 5-43970, for instance).
However, since the core material of the clad sheet described above consists of JIS 7000 alloy containing Zn in large quantity (normally 4% by weight or more), there is a great potential difference between the core material and the surface material. Thus, the core material preferentially melts in the etching process prior to the treatment with zincate, resulting in a difference in level (bump) between the core material and the surface material in some cases. When the difference in level between the core material and the surface material is large, a portion corresponding to the difference in level of the magnetic disk is caught by a shaft in mounting, and the surface material is easily exfoliated. Further, unless components of the core material and the surface material are properly combined together, the bonding strength resulting from clad rolling is not sufficient in some cases.
Namely, the conventional clad sheet described above raises problems in its clad rolling properties and in that the potential difference between the core material and the surface material causes the preferential melting of the core material in the etching process prior to the treatment with zincate.